Photoelectric conversion element

ABSTRACT

Provided is a photoelectric conversion element including: a first electrode having opaqueness to light and formed of a metal; a hole blocking layer provided on the first electrode; an electron transport layer provided on the hole blocking layer; a hole transport layer provided on the electron transport layer; and a second electrode provided on the hole transport layer and having transmissivity to light, wherein the hole blocking layer contains an oxide of the metal in the first electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 15/730,815, filed on Oct. 12, 2017, which is acontinuation application of International Application No.PCT/JP2016/063685, filed May 6, 2016, which claims priority to JapanesePatent Application No. 2015-095777, filed May 8, 2015. The contents ofthese applications are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to a photoelectric conversion element.

Description of the Related Art

In recent years, driving power for electronic circuits has beensignificantly reduced, and it has become possible to drive variouselectronic parts such as sensors with a weak power (of a microwattorder). Expected uses of sensors include application to stand-alonepower systems (energy harvesting elements) capable of generating andconsuming power instantly. Among such energy harvesting elements, solarcells (which are a kind of photoelectric conversion elements) aredrawing attention as elements capable of generating power at anywherethere is light. Small-sized energy harvesting elements can be disposedat various places, and when combined with secondary cells, can serve asprimary cells that need no replacement. Improvement in the powergeneration performance will enable a greater downsizing and an increasein the number of times sensor information can be transmitted wirelessly.

Among the solar cells, dye-sensitized solar cells proposed by Graetzelet al. from Swiss Federal Institute of Technology in Lausanne have beenreported to have high photoelectric conversion characteristics greaterthan or equal to photoelectric conversion characteristics of amorphoussilicon solar cells in environments under weak room light (see PanasonicTechnical Report, Vol. 56, No. 4 (2008) 87). Room light of, for example,LED lights and fluorescent lamps typically has illuminance of about from200 Lux through 1,000 Lux, and is light by far weaker than directsunlight (about 100,000 Lux). In many cases, the energy harvestingelements are installed on, for example, walls, but not directly underthe room light. In this case, light radiated to the energy harvestingelements becomes even weaker light. Walls in offices are at about 300Lux, and walls in work rooms are at about 500 Lux. Hence, a highphotoelectric conversion efficiency is desired even in environmentsunder weak light such as room light.

Meanwhile, existing dye-sensitized solar cells using electrolyticsolutions are at a risk of, for example, volatilization or leak of theelectrolytic solutions. Therefore, for practical use, it is desired toprovide the electrolytic solutions in the form of solids. For example,many reports have been made on solid dye-sensitized solar cells aspresented below.

(1) Solid dye-sensitized solar cells using inorganic semiconductors(see, for example, Semicond. Sci. Technol., 10 (1995) 1689)

(2) Solid dye-sensitized solar cells using low-molecular-weight organichole transport materials (see, for example, Japanese Unexamined PatentApplication Publication No. 11-144773, Synthetic Metals, 89 (1997) 215,and Nature, 398 (1998) 583)

(3) Solid dye-sensitized solar cells using conductive polymers (see, forexample, Japanese Unexamined Patent Application Publication No.2000-106223 and Chem. Lett., (1997) 471)

However, reports that have been made so far on solid dye-sensitizedsolar cells are merely reports on photoelectric conversion efficienciesunder artificial sunlight, but not on reports on photoelectricconversion efficiencies under, for example, room light. Especially,photoelectric conversion efficiencies in environments under weak light(from 50 Lux through 300 Lux) have not been reported at all.

SUMMARY OF THE INVENTION

According to one aspect of the present disclosure, a photoelectricconversion element includes a first electrode having opaqueness to lightand formed of a metal, a hole blocking layer provided on the firstelectrode, an electron transport layer provided on the hole blockinglayer, a hole transport layer provided on the electron transport layer,and a second electrode provided on the hole transport layer and havingtransmissivity to light. The hole blocking layer contains an oxide ofthe metal in the first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an example of a photoelectricconversion element of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure has an object to provide a photoelectricconversion element having a high photoelectric conversion efficiencyeven under weak irradiation light (from 50 Lux through 300 Lux) such asroom light.

The present disclosure can provide a photoelectric conversion elementhaving a high photoelectric conversion efficiency even under weakirradiation light (from 50 Lux through 300 Lux) such as room light.

(Photoelectric Conversion Element)

A photoelectric conversion element of the present disclosure includes afirst electrode having opaqueness to light and formed of a metal, a holeblocking layer provided on the first electrode, an electron transportlayer provided on the hole blocking layer, a hole transport layerprovided on the electron transport layer, and a second electrodeprovided on the hole transport layer and having transmissivity to light.The photoelectric conversion element further includes other layers asneeded.

<First Electrode>

The first electrode is not particularly limited and may be appropriatelyselected depending on the intended purpose, so long as the firstelectrode has non-transmissivity (opaqueness) to visible light and isformed of a metal.

The opaqueness to light means that transmittance of visible light islower than 50%.

The transmittance of visible light can be measured with, for example, anultraviolet-visible (UV-vis) spectroscopy method.

Examples of the material of the first electrode include silver,stainless steel, copper, titanium, and aluminium. One of these materialsmay be used alone or two or more of these materials may be used incombination. Among these materials, titanium is preferable.

The average thickness of the first electrode is preferably 100 nm orgreater, more preferably 1 micrometer or greater, and yet morepreferably 50 micrometers or greater.

In order for the first electrode to maintain a predetermined hardness,it is preferable to provide the first electrode on a substrate when theaverage thickness of the first electrode is 1 micrometer or less.Examples of the substrate include a glass plate, a transparent plasticplate, a transparent plastic film, and an inorganic transparent crystal.

When the average thickness of the first electrode is about 50micrometers, the first electrode can maintain hardness, and hence doesnot need a substrate.

<Hole Blocking Layer>

The hole blocking layer is provided in order to suppress a fall inelectric power due to contact of an electrolyte with an electrode andconsequent recombination between holes in the electrolyte and electronsin a surface of the electrode (so-called back electron transfer). Thehole blocking layer is formed also for the purpose of preventing anelectronic contact between the first electrode and the hole transportlayer.

The effect of the hole blocking layer is particularly remarkable insolid dye-sensitized solar cells. This is because a speed ofrecombination (back electron transfer) between holes in hole transportmaterials and electrons in surfaces of electrodes is higher in soliddye-sensitized solar cells using, for example, organic hole transportmaterials than in wet dye-sensitized solar cells using electrolyticsolutions.

The hole blocking layer contains an oxide of the metal in the firstelectrode.

The material of the hole blocking layer is not particularly limited andmay be appropriately selected depending on the intended purpose so longas the material is an oxide of the metal in the first electrode and istransparent (transmissive) to visible light. Examples of the material ofthe hole blocking layer includes titanium oxide, copper oxide, andaluminium oxide. Among these materials, titanium oxide is preferable.

The method for producing the hole blocking layer is not particularlylimited and may be appropriately selected depending on the intendedpurpose. In order to suppress loss current under room light, a highinternal resistance is needed, and a film forming method is important.

Examples of typical methods for producing the hole blocking layerinclude a sol-gel method, which is wet film formation, which howeverresults in a low film density to make it impossible to suppress losscurrent sufficiently. Hence, dry film formation such as a sputteringmethod is preferable and can provide a sufficiently high film density tomake it possible to suppress loss current.

As the sputtering method, a reactive sputtering method by an oxygen gasusing a target formed of a metal is preferable.

When the first electrode is heated in an oxygen atmosphere at from 700degrees C. through 1,100 degrees C., the metal in the surface of thefirst electrode is reformed to a metal oxide. This makes it possible tocoat the surface of the first electrode with a metal oxide film. Themetal oxide film is more excellent in the characteristic of interfacebonding with the first electrode and can hence improve thecharacteristic of electron injection into the first electrode needed.

The average thickness of the hole blocking layer is not particularlylimited, may be appropriately selected depending on the intendedpurpose, and is preferably 10 nm or greater but 1 micrometer or less.

<Electron Transport Layer>

The electron transport layer is a porous layer provided on the holeblocking layer, and may contain a single layer or multiple layers.

In the case of multiple layers, it is possible to form multiple layersby coating dispersion liquids of semiconductor particles havingdifferent particle diameters, or it is also possible to form multiplelayers by coating different kinds of semiconductors or different resinor additive compositions.

When a sufficient film thickness is not obtained with one coating,coating of multiple layers is an effective means.

Typically, an amount of a photosensitizing compound supported by theelectron transport layer per a unit projected area increases as athickness of the electron transport layer is increased, leading to anincrease in a light capture rate. However, this also increases adistance to which injected electrons diffuse, to increase loss due torecombination of charges.

Hence, the average thickness of the electron transport layer ispreferably 100 nm or greater but 100 micrometers or less.

The semiconductor particles are not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe semiconductor particles include element semiconductors such assilicon and germanium, compound semiconductors represented bychalcogenides of metals, and compounds having a perovskite structure.One of these kinds of semiconductor particles may be used alone or twoor more of these kinds of semiconductor particles may be used incombination.

Examples of the chalcogenides of metals include: oxides of titanium,tin, zinc, iron, tungsten, zirconium, hafnium, strontium, indium,cerium, yttrium, lanthanum, vanadium, niobium, and tantalum; sulfides ofcadmium, zinc, lead, silver, antimony, and bismuth; selenides of cadmiumand lead; and telluride of cadmium.

Examples of the compound semiconductors include: phosphides of, forexample, zinc, gallium, indium, and cadmium; gallium arsenide;copper-indium-selenide; and copper-indium-sulfide.

Examples of the compounds having a perovskite structure includestrontium titanate, calcium titanate, sodium titanate, barium titanate,and potassium niobate.

Among these semiconductor particles, oxide semiconductors arepreferable, and titanium oxide, zinc oxide, tin oxide, and niobium oxideare more preferable.

A crystal form of the semiconductor is not particularly limited, may beappropriately selected depending on the intended purpose, and may bemonocrystalline, polycrystalline, or amorphous.

The size of the semiconductor particles is not particularly limited andmay be appropriately selected depending on the intended purpose. Theaverage particle diameter of the primary particles of the semiconductorparticles is preferably 1 nm or greater but 100 nm or less and morepreferably 5 nm or greater but 50 nm or less.

It is also possible to improve photoelectric conversion efficiency basedon an incident-light-scattering effect obtained by mixing or laminatingsemiconductor particles having a greater average particle diameter.

The average particle diameter of the semiconductor particles ispreferably 50 nm or greater but 500 nm or less.

A method for producing the electron transport layer is not particularlylimited and may be appropriately selected depending on the intendedpurpose. Examples of the method include a method for forming a thin filmin vacuum, such as sputtering, and a wet film forming method. Whenproduction costs and other factors are taken into consideration, amongthese methods, the wet film forming method is preferable, and a methodof preparing a paste in which powder or sol of the semiconductorparticles is dispersed, and coating a substrate with the paste is morepreferable.

In using the wet film forming method, a coating method is notparticularly limited and may be performed according to a known method.For example, examples of the known method include a dipping method, aspraying method, a wire bar method, a spin coating method, a rollercoating method, a blade coating method, and a gravure coating method.Moreover, as wet printing methods, various methods such as letterpress,offset, gravure, intaglio, rubber plate, and screen printing may beused.

In producing a dispersion liquid of the semiconductor particles bymechanical pulverization or using a mill, it is possible to form thedispersion liquid by dispersing at least semiconductor particles aloneor a mixture of semiconductor particles and a resin in water or anorganic solvent.

The resin is not particularly limited and may be appropriately selecteddepending on the intended purpose. Examples of the resin includepolymers or copolymers of vinyl compounds based on, for example,styrene, vinyl acetate, acrylic ester, and methacrylic ester, siliconeresins, phenoxy resins, polysulfone resins, polyvinyl butyral resins,polyvinyl formal resins, polyester resins, cellulose ester resins,cellulose ether resins, urethane resins, phenol resins, epoxy resins,polycarbonate resins, polyallylate resins, polyamide resins, andpolyimide resins. One of these resins may be used alone or two or moreof these resins may be used in combination.

The solvent in which the semiconductor particles are dispersed is notparticularly limited and may be appropriately selected depending on theintended purpose. Examples of the solvent include: water; alcohol-basedsolvents; ketone-based solvents; ester-based solvents; ether-basedsolvents; amide-based solvents; halogenated hydrocarbon-based solvents;and hydrocarbon-based solvents. One of these solvents may be used aloneor two or more of these solvents may be used in combination.

Examples of the alcohol-based solvents include methanol, ethanol,isopropyl alcohol, and α-terpineol.

Examples of the ketone-based solvents include acetone, methyl ethylketone, and methyl isobutyl ketone.

Examples of the ester-based solvents include ethyl formate, ethylacetate, and n-butyl acetate.

Examples of the ether-based solvents include diethyl ether,dimethoxyethane, tetrahydrofuran, dioxolane, and dioxane.

Examples of the amide-based solvents include N,N-dimethylformamide,N,N-dimethylacetamide, and N-methyl-2-pyrrolidone.

Examples of the halogenated hydrocarbon-based solvents includedichloromethane, chloroform, bromoform, methyl iodide, dichloroethane,trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene,fluorobenzene, bromobenzene, iodobenzene, and 1-chloronaphthalene.

Examples of the hydrocarbon-based solvents include n-pentane, n-hexane,n-octane, 1,5-hexadiene, cyclohexane, methylcyclohexane, cyclohexadiene,benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, andcumene.

For prevention of reaggregation of particles, for example, an acid suchas hydrochloric acid, nitric acid, and acetic acid, a surfactant such aspolyoxyethylene (10) octylphenyl ether, and a chelate agent such asacetylacetone, 2-aminoethanol, and ethylene diamine may be added to thedispersion liquid of the semiconductor particles or to the paste of thesemiconductor particles obtained by, for example, a sol-gel method.

Furthermore, adding a thickener with a view to improving a film formingproperty is an effective means.

Examples of the thickener include polyethylene glycols, polyvinylalcohols, and ethyl cellulose.

It is preferable to subject the semiconductor particles after coated tofiring, microwave irradiation, electron beam irradiation, or laser lightirradiation in order to provide an electronic contact between theparticles, improve a film strength, and improve close adhesiveness withthe substrate. One of these treatments may be applied alone or two ormore kinds of these treatments may be applied in combination.

In the firing, a firing temperature is not limited to a particular rangeand may be appropriately selected depending on the intended purpose.However, the firing temperature is preferably 30 degrees C. or higherbut 700 degrees C. or lower and more preferably 100 degrees C. or higherbut 600 degrees C. or lower because the resistance of the substrate maybecome high or the substrate may melt if the temperature is excessivelyhigh. A firing time is also not particularly limited, may beappropriately selected depending on the intended purpose, and ispreferably 10 minutes or longer but 10 hours or shorter.

The microwave irradiation may be given from a side at which the electrontransport layer is formed or from a back side.

An irradiation time is not particularly limited, may be appropriatelyselected depending on the intended purpose, and is preferably within 1hour.

After firing, chemical plating using an aqueous solution of titaniumtetrachloride or a mixture solution of titanium tetrachloride with anorganic solvent or an electrochemical plating treatment using a titaniumtrichloride aqueous solution may be performed with a view to increasinga surface area of the semiconductor particles and increasing efficiencyof electron injection from a photosensitizing compound into thesemiconductor particles.

A porous state is formed in the film obtained by depositing thesemiconductor particles having a diameter of several tens of nanometersby, for example, sintering. This nanoporous structure has an extremelylarge surface area. The surface area can be expressed by a roughnessfactor.

The roughness factor is a value representing an actual area inside theporous texture relative to an area of the semiconductor particles coatedon the substrate. Hence, a greater roughness factor is more preferable.However, considering the relationship with the thickness of the electrontransport layer, the roughness factor is preferably 20 or greater.

—Photosensitizing Compound—

In the present disclosure, in order to further improve the conversionefficiency, a photosensitizing compound is adsorbed to the surface ofthe electron transport semiconductor serving as the electron transportlayer.

The photosensitizing compound is not particularly limited and may beappropriately selected depending on the intended purpose so long as thephotosensitizing compound is a compound optically excitable byexcitation light used. Examples of the photosensitizing compoundinclude: metal complex compounds described in, e.g., JapaneseTranslation of PCT International Application Publication No.JP-T-07-500630, and Japanese Unexamined Patent Application PublicationNos. 10-233238, 2000-26487, 2000-323191, and 2001-59062; coumarincompounds described in, e.g., Japanese Unexamined Patent ApplicationPublication Nos. 10-93118, 2002-164089 and 2004-95450, and J. Phys.Chem. C, 7224, Vol. 111 (2007); polyene compounds described in, e.g.,Japanese Unexamined Patent Application Publication No. 2004-95450 andChem. Commun., 4887 (2007); indoline compounds described in, e.g.,Japanese Unexamined Patent Application Publication Nos. 2003-264010,2004-63274, 2004-115636, 2004-200068, and 2004-235052, J. Am. Chem.Soc., 12218, Vol. 126 (2004), Chem. Commun., 3036 (2003), and Angew.Chem. Int. Ed., 1923, Vol. 47 (2008); thiophene compounds described in,e.g., J. Am. Chem. Soc., 16701, Vol. 128 (2006) and J. Am. Chem. Soc.,14256, Vol. 128 (2006); cyanine dyes described in, e.g., JapaneseUnexamined Patent Application Publication Nos. 11-86916, 11-214730,2000-106224, 2001-76773, and 2003-7359; merocyanine dyes described in,e.g., Japanese Unexamined Patent Application Publication Nos. 11-214731,11-238905, 2001-52766, 2001-76775, and 2003-7360; 9-arylxanthenecompounds described in, e.g., Japanese Unexamined Patent ApplicationPublication Nos. 10-92477, 11-273754, 11-273755, and 2003-31273;triarylmethane compounds described in, e.g., Japanese Unexamined PatentApplication Publication Nos. 10-93118 and 2003-31273; and phthalocyaninecompounds and porphyrin compounds described in, e.g., JapaneseUnexamined Patent Application Publication Nos. 09-199744, 10-233238,11-204821 and 11-265738, J. Phys. Chem., 2342, Vol. 91 (1987), J. Phys.Chem. B, 6272, Vol. 97 (1993), Electroanal. Chem., 31, Vol. 537 (2002),Japanese Unexamined Patent Application Publication No. 2006-032260, J.Porphyrins Phthalocyanines, 230, Vol. 3 (1999), Angew. Chem. Int. Ed.,373, Vol. 46 (2007), and Langmuir, 5436, Vol. 24 (2008). One of thesephotosensitizing compounds may be used alone or two or more of thesephotosensitizing compounds may be used in combination. Among thesephotosensitizing compounds, metal complex compounds, coumarin compounds,polyene compounds, indoline compounds, and thiophene compounds arepreferable.

Preferable examples of the photosensitizing compound include D131represented by structural formula (2) below and available fromMitsubishi Paper Mills Limited and D102 represented by structuralformula (3) below and available from Mitsubishi Paper Mills Limited.

A compound represented by general formula (2) below is also preferableas the photosensitizing compound.

In general formula (2), R represents a substituted or unsubstitutedalkyl group.

It is preferable that R in general formula (2) be an alkyl group or acarboxylic acid group-substituted alkyl group.

Examples of specific exemplary compounds of general formula (2) include,but are not limited to, D358 represented by structural formula (4) belowand available from Mitsubishi Paper Mills Limited.

Examples of a method for adsorbing the photosensitizing compound to theelectron transport layer include a method of immersing an electroncollecting electrode including the semiconductor particles in a solutionor dispersion liquid of the photosensitizing compound, and a method ofcoating the electron transport layer with the solution or the dispersionliquid of the photosensitizing compound to adsorb the photosensitizingcompound.

In the case of the method of immersing an electron collecting electrodeincluding the semiconductor particles in a solution or dispersion liquidof the photosensitizing compound, for example, an immersing method, adipping method, a roller method, and an air knife method may be used.

In the case of the method of coating the electron transport layer withthe solution or the dispersion liquid of the photosensitizing compoundto adsorb the photosensitizing compound, for example, a wire bar method,a slide hopper method, an extrusion method, a curtain method, a spinningmethod, and a spraying method may be used.

The photosensitizing compound may be adsorbed under a supercriticalfluid of, for example, carbon dioxide.

In adsorbing the photosensitizing compound, a condensing agent may beused in combination.

The condensing agent may be any of: a substance that is assumed tocatalyze physical or chemical binding of the photosensitizing compoundand the electron transport compound with a surface of an inorganicsubstance; and a substance that acts stoichiometrically to cause achemical equilibrium to move in an advantageous manner.

Furthermore, thiol or a hydroxy compound may be added as a condensingaid.

A solvent in which the photosensitizing compound is dissolved ordispersed is not particularly limited and may be appropriately selecteddepending on the intended purpose. Examples of the solvent includewater, alcohol-based solvents, ketone-based solvents, ester-basedsolvents, ether-based solvents, amide-based solvents, halogenatedhydrocarbon-based solvents, and hydrocarbon-based solvents. One of thesesolvents may be used alone or two or more of these solvents may be usedin combination.

Examples of the alcohol-based solvents include methanol, ethanol, andisopropyl alcohol.

Examples of the ketone-based solvents include acetone, methyl ethylketone, and methyl isobutyl ketone.

Examples of the ester-based solvents include ethyl formate, ethylacetate, and n-butyl acetate.

Examples of the ether-based solvents include diethyl ether,dimethoxyethane, tetrahydrofuran, dioxolane, and dioxane.

Examples of the amide-based solvents include N,N-dimethylformamide,N,N-dimethylacetamide, and N-methyl-2-pyrrolidone.

Examples of the halogenated hydrocarbon-based solvents includedichloromethane, chloroform, bromoform, methyl iodide, dichloroethane,trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene,fluorobenzene, bromobenzene, iodobenzene, and 1-chloronaphthalene.

Examples of the hydrocarbon-based solvents include n-pentane, n-hexane,n-octane, 1,5-hexadiene, cyclohexane, methylcyclohexane, cyclohexadiene,benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, andcumene.

Some kinds of photosensitizing compounds act more effectively whenaggregation of the compound is suppressed. Hence, a deaggregating agentmay be used in combination.

The deaggregating agent is not particularly limited depending on the dyeto be used and may be appropriately selected depending on the intendedpurpose. Preferable examples of the deaggregating agent include steroidcompounds such as cholic acid and chenodeoxycholic acid, long-chainalkylcarboxylic acids, or long-chain alkylphosphonic acids.

The content of the deaggregating agent is preferably 0.01 parts by massor greater but 500 parts by mass or less and more preferably 0.1 partsby mass or greater but 100 parts by mass or less relative to 1 part bymass of the dye.

The temperature for adsorbing the photosensitizing compound or thephotosensitizing compound and the deaggregating agent is preferably −50degrees C. or higher but 200 degrees C. or lower.

The adsorption may be performed in a still state or under stirring.

A method for the stirring is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe method include a stirrer, a ball mill, a paint conditioner, a sandmill, an attritor, a disperser, and ultrasonic dispersion.

A time needed for the adsorption is preferably 5 seconds or longer but1,000 hours or shorter, more preferably 10 seconds or longer but 500hours or shorter, and yet more preferably 1 minute or longer but 150hours.

It is preferable to perform the adsorption in a dark place.

<Hole Transport Layer>

The hole transport layer contains a hole transport material, preferablycontains a basic compound and lithiumbis(trifluoromethanesulfonyl)imide, and further contains othercomponents as needed.

The hole transport layer may have a single-layer structure or alaminated structure formed of different compounds. In the case of thelaminated structure, it is preferable to use a polymer as a holetransport material in the hole transport layer near the secondelectrode. This is because use of a polymer having an excellent filmforming property can make the surface of the porous electron transportlayer smoother and can improve the photoelectric conversioncharacteristic.

Furthermore, it is difficult for a polymer to permeate the inside of theporous electron transport layer. This in turns makes the polymerexcellent in coating the surface of the porous electron transport layerand effective for preventing short circuiting when an electrode isprovided, leading to a higher performance.

—Hole Transport Material—

A hole transport material used in a single-layer structure is notparticularly limited and may be appropriately selected depending on theintended purpose. Examples of the hole transport material include:oxadiazole compounds presented in, e.g., Japanese Examined PatentPublication No. 34-5466; triphenylmethane compounds presented in, e.g.,Japanese Examined Patent Publication No. 45-555; pyrazoline compoundspresented in, e.g., Japanese Examined Patent Publication No. 52-4188;hydrazone compounds presented in, e.g., Japanese Examined PatentPublication No. 55-42380; oxadiazole compounds presented in, e.g.,Japanese Unexamined Patent Application Publication No. 56-123544;tetraarylbenzidine compounds presented in Japanese Unexamined PatentApplication Publication No. 54-58445; and stilbene compounds presentedin Japanese Unexamined Patent Application Publication No. 58-65440 orJapanese Unexamined Patent Application Publication No. 60-98437. One ofthese hole transport materials may be used alone or two or more of thesehole transport materials may be used in combination.

Among these hole transport materials, a hole transport material(2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene:spiro-OMeTAD) described in “J. Am. Chem. Soc., 133 (2011) 18042” and ahole transport material(2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene)described in “J. Am. Chem. Soc., 135 (2013) 7378” are preferable interms of photoelectric conversion efficiency. Spiro-OMeTAD isparticularly preferable. The spiro-OMeTAD is represented by structuralformula (1) below.

The spiro-OMeTAD has a high Hall mobility and includes two benzidineskeleton molecules that are bound with each other in a twisted state.Hence, the spiro-OMeTAD forms an electron cloud close to a sphericalshape and has a good hopping conductivity between the molecules, leadingto a more excellent photoelectric conversion characteristic. Thespiro-OMeTAD also has a high solubility, is soluble in various organicsolvents, and is amorphous (i.e., an amorphous substance having nocrystalline structure). Therefore, the spiro-OMeTAD is likely to bedensely filled in the porous electron transport layer and has propertiesuseful for a solid dye-sensitized solar cell. Moreover, the spiro-OMeTADdoes not have a light absorbing property at 450 nm or longer. Therefore,the spiro-OMeTAD can enable light to be efficiently absorbed into thephotosensitizing compound, and has a property useful for a soliddye-sensitized solar cell. A thickness of the hole transport layerformed of the spiro-OMeTAD is not limited. It is preferable that thehole transport layer have a structure of intruding into voids of theporous electron transport layer, and it is preferable that the holetransport layer have a thickness of preferably 0.01 micrometers orgreater and more preferably 0.1 micrometers or greater but 10micrometers or less on the electron transport layer.

A known hole transportable polymer is used as the polymer to be usednear the second electrode in the hole transport layer used in the formof the laminated structure.

Examples of the hole transportable polymer include polythiophenecompounds, polyphenylenevinylene compounds, polyfluorene compounds,polyphenylene compounds, polyarylamine compounds, and polythiadiazolecompounds. One of these hole transportable polymers may be used alone ortwo or more of these hole transportable polymers may be used incombination.

Examples of the polythiophene compounds includepoly(3-n-hexylthiophene), poly(3-n-octyloxythiophene),poly(9,9′-dioctyl-fluorene-co-bithiophene), poly(3,3′″-didodecyl-quarterthiophene), poly(3,6-dioctylthieno[3,2-b]thiophene),poly(2,5-bis(3-decylthiophen-2-yl)thieno[3,2-b]thiophene),poly(3,4-didecylthiophene-co-thieno[3,2-b]thiophene),poly(3,6-dioctylthieno[3,2-b]thiophene-co-thieno[3,2-b]thiophene),poly(3,6-dioctylthieno[3,2-b]thiophene-co-thiophene), andpoly(3,6-dioctylthieno[3,2-b]thiophene-co-bithiophene).

Examples of the polyphenylenevinylene compounds includepoly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene],poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], andpoly[(2-methoxy-5-(2-ethylphexyloxy)-1,4-phenylenevinylene)-co-(4,4′-biphenylene-vinylene)].

Examples of the polyfluorene compounds includepoly(9,9′-didodecylfluorenyl-2,7-diyl),poly[(9,9-dioctyl-2,7-divinylenefluorene)-alt-co-(9,10-anthracene)],poly[(9,9-dioctyl-2,7-divinylenefluorene)-alt-co-(4,4′-biphenylene)],poly[(9,9-dioctyl-2,7-divinylenefluorene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)],and poly[(9,9-dioctyl-2,7-diyl)-co-(1,4-(2,5-dihexyloxy)benzene)].

Examples of the polyphenylene compounds includepoly[2,5-dioctyloxy-1,4-phenylene] andpoly[2,5-di(2-ethylhexyloxy-1,4-phenylene].

Examples of the polyarylamine compounds includepoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(N,N′-diphenyl)-N,N′-di(p-hexylphenyl)-1,4-diaminobenzene],poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(N,N′-bis(4-octyloxyphenyl)benzidine-N,N′-(1,4-diphenylene)],poly[(N,N′-bis(4-octyloxyphenyl)benzidine-N,N′-(1,4-diphenylene)],poly[(N,N′-bis(4-(2-ethylhexyloxy)phenyl)benzidine-N,N′-(1,4-diphenylene)],poly[phenylimino-1,4-phenylenevinylene-2,5-dioctyloxy-1,4-phenylenevinylene-1,4-phenylene],poly[p-tolylimino-1,4-phenylenevinylene-2,5-di(2-ethylhexyloxy)-1,4-phenylenevinylene-1,4-phenylene],and poly[4-(2-ethylhexyloxy)phenylimino-1,4-biphenylene].

Examples of the polythiadiazole compounds includepoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo(2,1′,3)thiadiazole]and poly(3,4-didecylthiophene-co-(1,4-benzo(2,1′,3)thiadiazole).

Among these hole transportable polymers, the polythiophene compounds andthe polyarylamine compounds are particularly preferable in terms ofcarrier mobility and ionization potential.

With a view to improving conductivity, an oxidizing agent for changingpart of the hole transport material to a radical cation may be added.

Examples of the oxidizing agent include tris(4-bromophenyl)aminiumhexachloroantimonate, silver hexafluoroantimonate, nitrosoniumtetrafluoroborate, silver nitrate, and cobalt complex-based compounds.

There is no need that the whole of the organic hole transport materialbe oxidized by addition of the oxidizing agent. Only part of the organichole transport material needs to be oxidized. It is optional whether theadded oxidizing agent is removed or not to the outside of the systemafter addition.

—Basic Compound—

By the hole transport layer containing a basic compound represented bygeneral formula (1) below, a high open circuit voltage can be obtained.Furthermore, an internal resistance in the photoelectric conversionelement rises to enable reduction of loss current under weak light suchas room light. Hence, it is possible to obtain an open circuit voltagehigher than obtained with an existing basic compound.

In general formula (1), R₁ and R₂ represent a substituted orunsubstituted alkyl group or aromatic hydrocarbon group and may beidentical or different, and R₁ and R₂ may bind with each other to form asubstituted or unsubstituted heterocyclic group containing a nitrogenatom.

Hitherto, basic compounds of general formula (1) such as compounds Nos.2-1 and 2-3 have been known to be used as basic compounds in iodineelectrolytic solution-type dye-sensitized solar cells. These compoundshave been reported to provide a high open circuit voltage, but tosignificantly reduce a short-circuiting current density and considerablyworsen a photoelectric conversion characteristic.

The hole transport layer uses the hole transport material and isdifferent from a hole transport model based on, for example, the iodineelectrolytic solution. Hence, reduction of a short-circuiting currentdensity is low and a high open circuit voltage can be obtained, to makeit possible to obtain an excellent photoelectric conversioncharacteristic. Furthermore, it was possible to verify that aparticularly outstanding excellent performance was exhibited inphotoelectric conversion under weak light such as room light. Thisphotoelectric conversion is a scarcely reported case.

Specific exemplary compounds of general formula (1) will be presentedbelow. However, these compounds are non-limiting examples. Note that“Nikkaji No.” presented below indicates a number in Japan ChemicalSubstance Dictionary and is based on an organic compound databaseadministered by Japan Science and Technology Agency.

—Method for Synthesizing Basic Compound Represented by General Formula(1)—

As a method for synthesizing the basic compound represented by generalformula (1), there is the following easy synthesizing route, which isreported in “J. Org. Chem., 67 (2002) 3029”.

In general formula (1), R₁ and R₂ represent a substituted orunsubstituted alkyl group or aromatic hydrocarbon group and may beidentical or different. R₁ and R₂ may bind with each other to form asubstituted or unsubstituted heterocyclic group containing a nitrogenatom. X represents a halogen.

The amount of the basic compound represented by general formula (1) tobe added in the hole transport layer is preferably 1 part by mass orgreater but 30 parts by mass or less and more preferably 10 parts bymass or greater but 20 parts by mass or less relative to 100 parts bymass of the hole transport material.

By the basic compound represented by general formula (1) beingcontained, an internal resistance in the photoelectric conversionelement becomes even higher, to enable reduction of loss current underweak light (from 50 Lux through 300 Lux) such as room light.

The hole transport layer can contain a lithium compound such as lithiumbis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonylimide, and lithium diisopropylimide. Among these lithiumcompounds, lithium bis(trifluoromethanesulfonyl)imide is preferable.

—Other Components—

The other components are not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe other components include metal iodides, iodine salts of quaternaryammonium compounds, metal bromides, bromine salts of quaternary ammoniumcompounds, metal chlorides, metal acetates, metal sulfates, metalcomplexes, sulfur compounds, ionic liquids described in Inorg. Chem. 35(1996) 1168, basic compounds, and 1-n-hexyl-3-methyl imidazoliniumbis(trifluoromethylsulfonyl)imide.

Examples of the metal iodides include iodine, lithium iodide, sodiumiodide, potassium iodide, cesium iodide, calcium iodide, copper iodide,iron iodide, and silver iodide.

Examples of the iodine salts of quaternary ammonium compounds includetetraalkylammonium iodide and pyridinium iodide.

Examples of the metal bromides include lithium bromide, sodium bromide,potassium bromide, cesium bromide, and calcium bromide.

Examples of the bromine salts of quaternary ammonium compounds includetetraalkylammonium bromide and pyridinium bromide.

Examples of the metal chlorides include copper chloride and silverchloride.

Examples of the metal acetates include copper acetate, silver acetate,and palladium acetate.

Examples of the metal sulfates include copper sulfate and zinc sulfate.

Examples of the metal complexes include ferrocyanate-ferricyanate andferrocene-ferricinium ion.

Examples of the sulfur compounds include polysodium sulfide and alkylthiol-alkyl disulfide.

Examples of the ionic liquids described in Inorg. Chem. 35 (1996) 1168include viologen dyes, hydroquinone, etc.,1,2-dimethyl-3-n-propylimidazolinium iodide,1-methyl-3-n-hexylimidazolinium iodide,1,2-dimethyl-3-ethylimidazoliumtrifluoromethane sulfonic acid salt,1-methyl-3-butylimidazoliumnonafluorobutyl sulfonic acid salt, and1-methyl-3-ethylimidazoliumbis(trifluoromethyl)sulfonylimide.

Examples of the basic compounds include pyridine, 4-t-butylpyridine, andbenzimidazole.

The hole transport layer may be formed directly on the electrontransport layer supporting the photosensitizing compound.

A method for producing the hole transport layer is not particularlylimited and may be appropriately selected depending on the intendedpurpose. Examples of the method include a method for forming a thin filmin vacuum, such as vacuum vapor deposition, and a wet film formingmethod. Of these methods, considering production costs and otherfactors, the wet film forming method is preferable, and a method forcoating the electron transport layer with the hole transport layer ismore preferable.

The coating method is not particularly limited and may be appropriatelyselected depending on the intended purpose. Examples of the coatingmethod include a dipping method, a spraying method, a wire bar method, aspin coating method, a roller coating method, a blade coating method,and a gravure coating method. As wet printing methods, various methodssuch as letterpress, offset, gravure, intaglio, rubber plate, and screenprinting can be used. Film formation may be performed under asupercritical fluid or a subcritical fluid having a temperature/pressurelower than a critical point.

The supercritical fluid is not particularly limited and may beappropriately selected depending on the intended purpose so long as thesupercritical fluid exists as a non-condensable high-density fluid intemperature and pressure ranges higher than a limit (critical point)until which a gas and a liquid can coexist, and even when compressed,does not condense but is maintained at higher than or equal to acritical temperature and higher than or equal to a critical pressure.However, a supercritical fluid having a low critical temperature ispreferable.

Examples of the supercritical fluid include carbon monoxide, carbondioxide, ammonia, nitrogen, water, alcohol-based solvents such asmethanol, ethanol, and n-butanol, hydrocarbon-based solvents such asethane, propane, 2,3-dimethylbutane, benzene, and toluene, halogen-basedsolvents such as methylene chloride and chlorotrifluoromethane, andether-based solvents such as dimethyl ether. One of these supercriticalfluids may be used alone or two or more of these supercritical fluidsmay be used in combination.

Among these supercritical fluids, carbon dioxide is particularlypreferable because carbon dioxide has a critical pressure of 7.3 MPa anda critical temperature of 31 degrees C., and hence can form asupercritical state easily and is incombustible and easy to handle.

The subcritical fluid is not particularly limited and may beappropriately selected depending on the intended purpose so long as thesubcritical fluid exists as a high-pressure liquid in temperature andpressure ranges near critical points.

The compounds raised above as examples of the supercritical fluid canalso be used favorably as the subcritical fluid.

A critical temperature and a critical pressure of the supercriticalfluid are not particularly limited and may be appropriately selecteddepending on the intended purpose. However, the critical temperature ispreferably −273 degrees C. or higher but 300 degrees C. or lower andmore preferably 0 degrees C. or higher but 200 degrees C. or lower.

In addition to the supercritical fluid and the subcritical fluid, anorganic solvent and an entrainer may further be used in combination.Addition of an organic solvent and an entrainer makes it easier toadjust solubility to the supercritical fluid.

The organic solvent is not particularly limited and may be appropriatelyselected depending on the intended purpose. Examples of the organicsolvent include ketone-based solvents, ester-based solvents, ether-basedsolvents, amide-based solvents, halogenated hydrocarbon-based solvents,and hydrocarbon-based solvents.

Examples of the ketone-based solvents include acetone, methyl ethylketone, and methyl isobutyl ketone.

Examples of the ester-based solvents include ethyl formate, ethylacetate, and n-butyl acetate.

Examples of the ether-based solvents include diisopropyl ether,dimethoxyethane, tetrahydrofuran, dioxolane, and dioxane.

Examples of the amide-based solvents include N,N-dimethylformamide,N,N-dimethylacetamide, and N-methyl-2-pyrrolidone.

Examples of the halogenated hydrocarbon-based solvents includedichloromethane, chloroform, bromoform, methyl iodide, dichloroethane,trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene,fluorobenzene, bromobenzene, iodobenzene, and 1-chloronaphthalene.

Examples of the hydrocarbon-based solvents include n-pentane, n-hexane,n-octane, 1,5-hexadiene, cyclohexane, methylcyclohexane, cyclohexadiene,benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, andcumene.

In the present disclosure, a press process step may be performed afterthe hole transport layer is provided over the first electrode over whichthe electron transport layer coated with the photosensitizing compoundis provided.

It is considered that the press process makes close adhesion of theorganic hole transport material with the porous electrode stronger toimprove efficiency.

A method for the press process is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe method include a press forming method using a flat plate representedby an IR tablet molding machine, and a roll press method using, forexample, a roller.

A pressure is preferably 10 kgf/cm² or higher and more preferably 30kgf/cm² or higher. A time for which the press process is performed isnot particularly limited, and is preferably within 1 hour. Heat may beapplied during the press process.

In the press process, a release material may be interposed between apress machine and the electrode.

The release material is not particularly limited and may beappropriately selected depending on the intended purpose. Examples ofthe release material include fluororesins such aspolytetrafluoroethylene, polychlorotrifluoroethylene,tetrafluoroethylene-hexafluoropropylene copolymers, perfluoroalkoxyfluoride resins, polyvinylidene fluoride, ethylene-tetrafluoroethylenecopolymers, ethylene-chlorotrifluoroethylene copolymers, and polyvinylfluoride. One of these release materials may be used alone or two ormore of these release materials may be used in combination.

After the press process step, a metal oxide layer may be providedbetween the hole transport layer and the second electrode, before thesecond electrode is provided.

Examples of the metal oxide layer include molybdenum oxide, tungstenoxide, vanadium oxide, and nickel oxide. Among these metal oxide layers,molybdenum oxide is preferable.

A method for providing the metal oxide layer on the hole transport layeris not particularly limited and may be appropriately selected dependingon the intended purpose. Examples of the method include methods forforming a thin film in vacuum, such as sputtering and vacuum vapordeposition, and a wet film forming method.

As the wet film forming method, a method of preparing a paste in whichpowder or sol of the metal oxide is dispersed, and coating the holetransport layer with the paste is preferable.

In using the wet film forming method, a coating method is notparticularly limited and may be performed according to a known method.

Examples of the known method include a dipping method, a sprayingmethod, a wire bar method, a spin coating method, a roller coatingmethod, a blade coating method, and a gravure coating method.

Moreover, as wet printing methods, various methods such as letterpress,offset, gravure, intaglio, rubber plate, and screen printing may beused.

The average thickness of the metal oxide layer is not particularlylimited, may be appropriately selected depending on the intendedpurpose, and is preferably 0.1 nm or greater but 50 nm or less and morepreferably 1 nm or greater but 10 nm or less.

<Second Electrode>

The second electrode is not particularly limited so long as the secondelectrode is a conductive film having transmissivity to light. Knownconductive films used in typical photoelectric conversion elements or,for example, liquid crystal panels may be used.

The transmissivity to light means that transmittance of visible light is50% or higher and more preferably 70% or higher.

The transmittance of visible light can be measured with, for example, anultraviolet-visible (UV-vis) spectroscopy method.

Examples of the second electrode include: metals such as platinum, gold,silver, copper, and aluminium, or nanowire of these metals; carbon-basedcompounds such as graphite, fullerene, carbon nanotube, and graphene;conductive metal oxides such as indium tin oxide, fluorine-doped tinoxide, and antimony-doped tin oxide; conductive polymers such aspolythiophene and polyaniline; and PEDOT/PSS. One of these materials maybe used alone or two or more of these materials may be used incombination.

Among these materials, polythiophene, PEDOT/PSS, and metal nanowire arepreferable.

With a metal wire of, for example, silver dispersed, a low electricresistance can be obtained while transmissivity to light is maintained.

With a view to lowering a resistance, for example, a metal lead line maybe used in combination. Examples of the material of the metal lead lineinclude metals such as aluminium, copper, silver, gold, platinum, andnickel. The metal lead line can be formed on the second electrode filmby, for example, screen printing and vapor deposition.

The thickness of the second electrode layer is not particularly limited.The second electrode needs to have transmissivity to light, and may usea single material or a mixture of 2 or more materials.

The second electrode is formed on the hole transport layer.

Formation of the second electrode by coating can be performed byappropriate methods such as coating, lamination, vapor deposition, CVD,and pasting on the hole transport layer, depending on the kind of thematerial used and the kind of the hole transport layer.

In order to operate as a dye-sensitized solar cell, at least one of thefirst electrode and the second electrode needs to be substantiallytransparent (transmissive to light). In the present disclosure, thesecond electrode side is transparent. A preferable manner is thatsunlight is made incident from the second electrode side. In this case,it is preferable to use a light-reflecting material at the firstelectrode side. Metals, glass on which a conductive oxide isvapor-deposited, plastics, and metallic thin films are preferable.

Providing an antireflection layer at a sunlight incident side is also aneffective means.

The configuration of the photoelectric conversion element will bedescribed with reference to FIG. 1. FIG. 1 is an example of across-sectional view of a photoelectric conversion element and a solarcell.

The embodiment illustrated in FIG. 1 is a configuration example in whicha first electrode 2 is formed on a substrate 1, a hole blocking layer 3is formed on the first electrode 2, an electron transport layer 4 isformed on the hole blocking layer 3, a photosensitizing compound 5 isadsorbed to the electron transport material in the electron transportlayer 4, and a hole transport layer 6 is interposed between the firstelectrode 2 and a second electrode 7 counter to the first electrode 2.The configuration example illustrated in FIG. 1 also includes lead lines8 and 9 provided in a manner to make the first electrode 2 and thesecond electrode 7 electrically continuous. Note that no substrate 1 isneeded when the first electrode 2 has hardness.

<Applications>

In the present disclosure, the photoelectric conversion element refersto an element configured to convert light energy to electric energy oran element configured to convert electric energy to light energy.Specific examples of the photoelectric conversion element include asolar cell and a photodiode.

The photoelectric conversion element of the present disclosure can beapplied to a power supply device when combined with, for example, acircuit board configured to control a generated current. Examples ofdevices utilizing the power supply device include a desk-top electroniccalculator and a wristwatch. In addition, the power supply deviceincluding the photoelectric conversion element of the present disclosurecan be applied to, for example, a portable phone, an electronicorganizer, and an electronic paper. The power supply device includingthe photoelectric conversion element of the present disclosure can alsobe used as an auxiliary power supply intended for extending acontinuously usable time of chargeable or dry cell-operated electricappliances.

EXAMPLES

Examples of the present disclosure will be described below. However, thepresent disclosure should not be construed as being limited to theExamples.

In Examples and Comparative Examples below, being transmissive to lightmeans that transmittance of visible light is 50% or higher, and beingopaque to light means that transmittance of visible light is lower than50%.

The transmittance of visible light was measured with V-660DS (availablefrom Jasco Corporation), which was an ultraviolet-visible (UV-vis)spectrometer.

Example 1 <Production of Titanium Oxide Semiconductor Electrode>

As a first electrode, a titanium foil (opaque) having a thickness of 50micrometers was subjected to a heating treatment in a firing furnace(available from Furutech Co., Ltd., FTM-1300G-400) in an oxygenatmosphere at 750 degrees C. for 30 minutes, to obtain a first electrodeon whose surface a dense hole blocking layer formed of titanium oxidewas formed.

Next, titanium oxide (available from Nippon Aerosil Co., Ltd., P90) (3g), acetylacetone (0.2 g), and a surfactant (available from Wako PureChemical Industries, Ltd., polyoxyethylene octylphenyl ether) (0.3 g)were subjected to a bead mill treatment for 12 hours together with water(5.5 g) and ethanol (1.0 g), to obtain a titanium oxide dispersionliquid.

Polyethylene glycol (#20,000, available from Wako Pure ChemicalIndustries, Ltd.) (1.2 g) was added to the obtained titanium oxidedispersion liquid, to produce a paste.

The obtained paste was coated on the hole blocking layer to have anaverage thickness of 1.5 micrometers, dried at room temperature, andthen fired in the air at 500 degrees C. for 30 minutes, to form a porouselectron transport layer. In the way described above, a titanium oxidesemiconductor electrode was produced.

<Production of Dye-Sensitized Solar Cell>

The titanium oxide semiconductor electrode was immersed in a sensitizingdye, which was D358 available from Mitsubishi Paper Mills Limited andrepresented by structural formula (4) below (0.5 mM, anacetonitrile/t-butanol (at a volume ratio of 1:1) solution), and left tostand still for 1 hour in a dark place, to adsorb the photosensitizingcompound.

Next, lithium bis(trifluoromethanesulfonyl)imide (with a solid contentof 1% by mass, available from Kanto Chemical Co., Inc.) (12.83 mg) andan exemplary basic compound No. 2-1 represented by a structural formulabelow (with a solid content of 1.4% by mass) (18.3 mg) were added to achlorobenzene solution (with a solid content of 14% by mass) (1.28 g) ofa hole transport material (available from Luminescence Technology Corp.,name: N,N,N′,N′-tetrakis(4-methoxyphenyl)benzidine, product number:LT-N212), to prepare a hole transport layer coating liquid.

Next, the hole transport layer coating liquid was spin-coated on thesemiconductor electrode supporting the photosensitizing compound, toform a hole transport layer.

Next, a film of a paste (transmissive) of PEDOT/PSS (available fromSigma-Aldrich Co., LLC, ORGACON™ EL-P-5015) was formed on the holetransport layer by screen printing, and heated and dried at 100 degreesC. for 30 minutes, to form a second electrode. In the way describedabove, a dye-sensitized solar cell was produced.

<Evaluation of Dye-Sensitized Solar Cell>

An open circuit voltage, a short-circuiting current density, and aphotoelectric conversion efficiency of the obtained dye-sensitized solarcell under white LED irradiation (300 Lux: 75 microwatts/cm², 100 Lux:25 microwatts/cm², and 50 Lux: 12.5 watts/cm²) were measured. Theresults are presented in Table 1-1 to Table 1-3.

The measurement was performed using a desk lamp CDS-90a (study mode)available from Cosmotechno. Co., Ltd. as the white LED, and a solar cellevaluation system AS-510-PV03 available from NF Corporation as anevaluator.

Example 2

A solar cell was produced in the same manner as in Example 1, exceptthat unlike in Example 1, N,N,N′,N′-tetrakis(4-methoxyphenyl)benzidine,which was the hole transport material, was changed to spiro-OMeTADrepresented by structural formula (1) (available from Merck KGaA, name:2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene,product number: SHT-263). An open circuit voltage, a short-circuitingcurrent density, and a photoelectric conversion efficiency were measuredin the same manner as in Example 1. The results are presented in Table1-1 to Table 1-3.

Example 3

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, the titanium foil as the first electrode waschanged to a metal foil (opaque) having a three-layer structure formedof titanium (with a thickness of 20 micrometers)/stainless steel (with athickness of 60 micrometers)/titanium (with a thickness of 20micrometers). An open circuit voltage, a short-circuiting currentdensity, and a photoelectric conversion efficiency were measured in thesame manner as in Example 1. The results are presented in Table 1-1 toTable 1-3.

Example 4

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, PEDOT/PSS as the second electrode was changedto a coating liquid (transmissive) of silver nanowire (available fromSigma-Aldrich Co., LLC, product number: 730777). An open circuitvoltage, a short-circuiting current density, and a photoelectricconversion efficiency were measured in the same manner as in Example 1.The results are presented in Table 1-1 to Table 1-3.

Example 5

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, lithium bis(trifluoromethanesulfonyl)imide forthe hole transport layer was changed to 1-n-hexyl-3-methyl imidazoliniumbis(trifluoromethylsulfonyl)imide. An open circuit voltage, ashort-circuiting current density, and a photoelectric conversionefficiency were measured in the same manner as in Example 1. The resultsare presented in Table 1-1 to Table 1-3.

Example 6

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, the exemplary basic compound No. 2-1 waschanged to an exemplary basic compound No. 2-3 represented by astructural formula below. An open circuit voltage, a short-circuitingcurrent density, and a photoelectric conversion efficiency were measuredin the same manner as in Example 1. The results are presented in Table1-1 to Table 1-3.

Example 7 <Production of Titanium Oxide Semiconductor Electrode>

By a reactive sputter method by an oxygen gas using a target formed ofmetal titanium, a first electrode formed of titanium (opaque) and havinga thickness of 100 nm and a dense hole blocking layer formed of titaniumoxide and having a thickness of 10 nm were formed on a glass substrate.

Next, titanium oxide (available from Nippon Aerosil Co., Ltd., P90) (3g), acetylacetone (0.2 g), and a surfactant (available from Wako PureChemical Industries, Ltd., polyoxyethylene octylphenyl ether) (0.3 g)were subjected to a bead mill treatment for 12 hours together with water(5.5 g) and ethanol (1.0 g).

Polyethylene glycol (#20,000, available from Wako Pure ChemicalIndustries, Ltd.) (1.2 g) was added to the obtained dispersion liquid,to produce a paste.

The obtained paste was coated on the hole blocking layer to have athickness of 1.5 micrometers, dried at room temperature, and fired inthe air at 500 degrees C. for 30 minutes, to form a porous electrontransport layer. In the way described above, a titanium oxidesemiconductor electrode was produced.

<Production of Dye-Sensitized Solar Cell>

The titanium oxide semiconductor electrode was immersed in a sensitizingdye, which was D358 available from Mitsubishi Paper Mills Limited andrepresented by structural formula (4) below (0.5 mM, anacetonitrile/t-butanol (at a volume ratio of 1:1) solution), and left tostand still for 1 hour in a dark place, to adsorb the photosensitizingcompound.

Next, lithium bis(trifluoromethanesulfonyl)imide (with a solid contentof 1% by mass, available from Kanto Chemical Co., Inc.) (12.83 mg) andan exemplary basic compound No. 2-1 represented by a structural formulabelow (with a solid content of 1.4% by mass) (18.3 mg) were added to achlorobenzene solution (with a solid content of 14% by mass) (1.28 g) ofa hole transport material, which was spiro-OMeTAD represented bystructural formula (1) (available from Merck KGaA, name:2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene,product number: SHT-263), to prepare a hole transport layer coatingliquid.

Next, the hole transport layer coating liquid was spin-coated on thesemiconductor electrode supporting the photosensitizing compound, toform a hole transport layer.

Next, a film of a paste (transmissive) of PEDOT/PSS (available fromSigma-Aldrich Co., LLC, ORGACON™ EL-P-5015) was formed on the holetransport layer by screen printing, and heated and dried at 100 degreesC. for 30 minutes, to produce a second electrode, to produce adye-sensitized solar cell was produced.

An open circuit voltage, a short-circuiting current density, and aphotoelectric conversion efficiency of the obtained dye-sensitized solarcell were measured in the same manner as in Example 1. The results arepresented in Table 1-1 to Table 1-3.

Comparative Example 1

A solar cell was produced in the same manner as in Example 1, exceptthat unlike in Example 1, the titanium foil (opaque) as the firstelectrode and the hole blocking layer formed of titanium oxide formed onthe titanium foil were changed to an ITO-based glass substrate(transmissive) and a dense hole blocking layer of titanium oxide formedon the ITO-based glass substrate by a reactive sputter method by anoxygen gas using a target formed of metal titanium. An open circuitvoltage, a short-circuiting current density, and a photoelectricconversion efficiency were measured in the same manner as in Example 1.The results are presented in Table 1-1 to Table 1-3.

Comparative Example 2

A solar cell was produced in the same manner as in Example 1, exceptthat unlike in Example 1, the titanium foil as the first electrode waschanged to a stainless steel foil (opaque), and a hole blocking layerformed of titanium oxide was formed on the stainless steel foil by areactive sputter method by an oxygen gas using a target formed of metaltitanium. An open circuit voltage, a short-circuiting current density,and a photoelectric conversion efficiency were measured in the samemanner as in Example 1. The results are presented in Table 1-1 to Table1-3.

Comparative Example 3

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, the titanium foil (opaque) as the firstelectrode and the hole blocking layer formed of titanium oxide formed onthe titanium foil were changed to an ITO-based glass substrate(transmissive) and a dense hole blocking layer formed of titanium oxideformed on the ITO-based glass substrate by a reactive sputter method byan oxygen gas using a target formed of metal titanium. An open circuitvoltage, a short-circuiting current density, and a photoelectricconversion efficiency were measured in the same manner as in Example 1.The results are presented in Table 1-1 to Table 1-3.

Comparative Example 4

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, the titanium foil as the first electrode waschanged to a stainless steel foil (opaque), and a hole blocking layerformed of titanium oxide was formed on the stainless steel foil by areactive sputter method by an oxygen gas using a target formed of metaltitanium. An open circuit voltage, a short-circuiting current density,and a photoelectric conversion efficiency were measured in the samemanner as in Example 1. The results are presented in Table 1-1 to Table1-3.

Comparative Example 5

A solar cell was produced in the same manner as in Example 2, exceptthat unlike in Example 2, PEDOT/PSS as the second electrode was changedto a silver film (opaque) having an average thickness of 100 nm byvacuum vapor deposition. An open circuit voltage, a short-circuitingcurrent density, and a photoelectric conversion efficiency were measuredin the same manner as in Example 1. The results are presented in Table1-1 to Table 1-3.

TABLE 1-1 300 Lux (75 microwatts/cm²) Short-circuiting PhotoelectricOpen circuit current density conversion voltage (V) (microampere/cm²)efficiency (%) Ex. 1 0.791 15.09 10.98 Ex. 2 0.855 21.58 18.94 Ex. 30.851 21.47 18.27 Ex. 4 0.849 21.87 19.56 Ex. 5 0.831 21.83 18.62 Ex. 60.857 21.68 19.08 Ex. 7 0.848 21.49 18.47 Comp. Ex. 1 0.795 12.53 8.63Comp. Ex. 2 0.794 9.87 6.79 Comp. Ex. 3 0.854 17.32 15.19 Comp. Ex. 40.849 12.91 11.25 Comp. Ex. 5 Unmeasurable

TABLE 1-2 100 Lux (25 microwatts/cm²) Short-circuiting PhotoelectricOpen circuit current density conversion voltage (V) (microampere/cm²)efficiency (%) Ex. 1 0.771 5.03 11.01 Ex. 2 0.839 7.21 19.12 Ex. 3 0.8317.19 18.40 Ex. 4 0.829 7.21 18.89 Ex. 5 0.819 7.22 17.50 Ex. 6 0.8407.25 19.24 Ex. 7 0.832 7.11 18.22 Comp. Ex. 1 0.769 4.17 9.12 Comp. Ex.2 0.767 3.29 7.17 Comp. Ex. 3 0.838 5.72 15.15 Comp. Ex. 4 0.831 4.2911.27 Comp. Ex. 5 Unmeasurable

TABLE 1-3 50 Lux (12.5 microwatts/cm²) Short-circuiting PhotoelectricOpen circuit current density conversion voltage (V) (microampere/cm²)efficiency (%) Ex. 1 0.758 2.51 10.81 Ex. 2 0.825 3.62 18.87 Ex. 3 0.8213.58 18.11 Ex. 4 0.819 3.61 18.69 Ex. 5 0.805 3.42 16.30 Ex. 6 0.8263.63 18.95 Ex. 7 0.821 3.58 17.16 Comp. Ex. 1 0.754 2.07 8.94 Comp. Ex.2 0.752 1.65 7.03 Comp. Ex. 3 0.825 2.87 14.96 Comp. Ex. 4 0.826 2.1810.23 Comp. Ex. 5 Unmeasurable

From the results of Table 1-1 to Table 1-3, Comparative Example 1 usingthe first electrode and second electrode having transmissivity to light,to result in a low light trapping effect, was lower than Example 1 inthe short-circuiting current density and the photoelectric conversionefficiency.

Comparative Example 2 using different kinds of metals in the firstelectrode and the hole blocking layer, to result in a poor electroninjection characteristic, was lower than Example 1 in the short-circuitcurrent density and the photoelectric conversion efficiency.

Comparative Example 3 using the first electrode and second electrodehaving transmissivity to light, to result in a low light trappingeffect, was lower than Example 2 in the short-circuiting currentdensity.

Comparative Example 4 using different kinds of metals in the firstelectrode and the hole blocking layer, to result in a poor electroninjection characteristic, was lower than Example 2 in theshort-circuiting current density.

Comparative Example 5 using the second electrode opaque to light, toresult in impossibility for light to reach the power generating layer,had no photoelectric conversion characteristic at all (unmeasurable).

As compared, the solar cells of Examples 1 to 7 all had an excellentphotoelectric conversion efficiency. Among these Examples, Examples 2 to7 using spiro-OMeTAD represented by structural formula (1) as the holetransport material achieved a particularly high photoelectric conversionefficiency.

As described above, it turned out that the photoelectric conversionelement of the present disclosure was able to obtain a goodphotoelectric conversion performance even under weak irradiation light(from 50 Lux through 300 Lux) such as room light.

Aspects of the present disclosure are as follows, for example.

<1> A photoelectric conversion element including:a first electrode having opaqueness to light and formed of a metal;a hole blocking layer provided on the first electrode;an electron transport layer provided on the hole blocking layer;a hole transport layer provided on the electron transport layer; anda second electrode provided on the hole transport layer and havingtransmissivity to light,wherein the hole blocking layer contains an oxide of the metal in thefirst electrode.<2> The photoelectric conversion element according to <1>, wherein theoxide of the metal in the hole blocking layer is obtained by subjectingthe first electrode to a heating treatment in an oxygen atmosphere.<3> The photoelectric conversion element according to <1>, wherein theoxide of the metal in the hole blocking layer is obtained by a reactivesputter method by an oxygen gas using a target formed of the metal.<4> The photoelectric conversion element according to any one of <1> to<3>,wherein the oxide of the metal in the hole blocking layer is titaniumoxide.<5> The photoelectric conversion element according to any one of <1> to<4>,wherein the hole transport layer contains a hole transport materialrepresented by structural formula (1) below,

<6> The photoelectric conversion element according to any one of <1> to<5>,wherein the first electrode contains at least one metal selected fromthe group consisting of stainless steel, copper, titanium, andaluminium.<7> The photoelectric conversion element according to any one of <1> to<6>,wherein the second electrode contains at least one selected from thegroup consisting of polythiophene and metal nanowire.<8> The photoelectric conversion element according to any one of <1> to<7>,wherein the hole transport layer contains lithiumbis(trifluoromethanesulfonyl)imide.<9> The photoelectric conversion element according to any one of <1> to<8>,wherein a photoelectric conversion efficiency of the photoelectricconversion element under irradiation with a white LED of 100 Lux is 11%or higher.<10> The photoelectric conversion element according to any one of <1> to<9>,wherein the photoelectric conversion element is used in an environmentunder weak light of from 50 Lux through 300 Lux.<11> The photoelectric conversion element according to any one of <1> to<10>,wherein an average thickness of the hole blocking layer is 10 nm orgreater but 1 micrometer or less.<12> The photoelectric conversion element according to any one of <1> to<11>,wherein the first electrode contains titanium.<13> The photoelectric conversion element according to any one of <1> to<12>,wherein the hole transport layer contains a basic compound representedby general formula (1) below,

where in general formula (1), R₁ and R₂ represent a substituted orunsubstituted alkyl group or aromatic hydrocarbon group and may beidentical or different, and R₁ and R₂ may bind with each other to form asubstituted or unsubstituted heterocyclic group containing a nitrogenatom.<14> The photoelectric conversion element according to any one of <1> to<13>,wherein the basic compound represented by general formula (1) is a basiccompound represented by any one of structural formulae below,

<15> The photoelectric conversion element according to any one of <1> to<14>,wherein the electron transport layer contains an electron transportmaterial to which a photosensitizing compound represented by generalformula (2) below is adsorbed,

where in general formula (2), R represents a substituted orunsubstituted alkyl group.<16> The photoelectric conversion element according to any one of <1> to<15>,wherein the electron transport material is at least one selected fromthe group consisting of titanium oxide, zinc oxide, tin oxide, andniobium oxide.<17> The photoelectric conversion element according to any one of <1> to<16>,wherein the hole transport layer contains an ionic liquid.<18> The photoelectric conversion element according to any one of <1> to<17>, including a metal oxide layer between the hole transport layer andthe second electrode.<19> The photoelectric conversion element according to any one of <7> to<18>,wherein the metal nanowire is silver nanowire.<20> The photoelectric conversion element according to any one of <1> to<19>,wherein the transmissivity to light means that transmittance of visiblelight is 50% or higher, and the opaqueness to light means thattransmittance of visible light is lower than 50%.

The photoelectric conversion element according to any one of <1> to <20>aims for solving the various problems in the related art and achievingthe object described below. That is, the photoelectric conversionelement has an object to provide a photoelectric conversion elementcapable of achieving a good photoelectric conversion performance evenunder weak irradiation light (from 50 Lux through 300 Lux) such as roomlight.

What is claimed is:
 1. A photoelectric conversion element comprising: afirst electrode formed of a metal; a hole blocking layer provided on thefirst electrode; an electron transport layer provided on the holeblocking layer; a hole transport layer provided on the electrontransport layer; and a second electrode provided on the hole transportlayer, wherein the hole blocking layer comprises an oxide of the metalin the first electrode, and the hole transport layer comprises a basiccompound of formula (1):

wherein R1 and R2 represent a substituted or unsubstituted alkyl groupor aromatic hydrocarbon group and may be identical or different, and R1and R2 may bind with each other to form a substituted or unsubstitutedheterocyclic group comprising a nitrogen atom.
 2. The photoelectricconversion element according to claim 1, wherein the oxide of the metalin the hole blocking layer is obtained by subjecting the first electrodeto a heating treatment in an oxygen atmosphere.
 3. The photoelectricconversion element according to claim 1, wherein the oxide of the metalin the hole blocking layer is obtained by a reactive sputter method byan oxygen gas using a target formed of the metal.
 4. The photoelectricconversion element according to claim 1, wherein the oxide of the metalin the hole blocking layer comprises titanium oxide.
 5. Thephotoelectric conversion element according to claim 1, wherein the holetransport layer comprises a hole transport material represented bystructural formula (1) below,


6. The photoelectric conversion element according to claim 1, whereinthe first electrode comprises at least one metal selected from the groupconsisting of stainless steel, copper, titanium, and aluminium.
 7. Thephotoelectric conversion element according to claim 1, wherein thesecond electrode comprises at least one selected from the groupconsisting of polythiophene and metal nanowire.
 8. The photoelectricconversion element according to claim 1, wherein the hole transportlayer comprises lithium bis(trifluoromethanesulfonyl)imide.
 9. Thephotoelectric conversion element according to claim 1, wherein thephotoelectric conversion element is used in an environment under weaklight of from 50 Lux through 300 Lux.
 10. The photoelectric conversionelement according to claim 1, wherein the first electrode is a layerdirectly on a substrate, wherein the substrate is selected from thegroup consisting of a glass plate, a transparent plastic plate, atransparent plastic film, and an inorganic transparent crystal.
 11. Aphotoelectric conversion element comprising: a first electrode; a holeblocking layer provided on the first electrode; an electron transportlayer provided on the hole blocking layer; a hole transport layerprovided on the electron transport layer; and a second electrodeprovided on the hole transport layer, wherein the hole transport layercomprises a basic compound of formula (1): wherein R1 and R2 represent asubstituted or unsubstituted alkyl group or aromatic hydrocarbon groupand may be identical or different, and R1 and R2 may bind with eachother to form a substituted or unsubstituted heterocyclic groupcomprising a nitrogen atom.